The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of particles such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in co-pending applications, U.S, Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO01/67170), U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO02/01280) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001, all of which are incorporated herein by reference. The improved EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent.
All of these EPDs may be driven by a passive matrix system. For a typical passive matrix system, there are row electrodes on the top side and column electrodes on the bottom side of the cells. The top row electrodes and the bottom column electrodes are perpendicular to each other. However, there are two well-known problems associated with EPDs driven by a passive matrix system: cross talk and cross bias. Cross talk occurs when the particles in a cell are biased by the electric field of a neighboring cell. FIG. 1 provides an example. The bias voltage of the cell A drives the positively charged particles towards the bottom of the cell. Since cell B has no voltage bias, the positively charged particles in cell B are expected to remain at the top of the cell. However, if the two cells, A and B, are close to each other, the top electrode voltage of cell B (30V) and the bottom electrode voltage of cell A (0V) create a cross talk electric field which forces some of the particles in cell B to move downwards. Widening the distance between adjacent cells may eliminate such a problem; but the distance may also reduce the resolution of the display.
The cross talk problem may be lessened if a cell has a significantly high threshold voltage. The threshold voltage, in the context of the present invention, is defined to be the maximum bias voltage that may be applied to a cell without causing movement of particles between two electrodes on opposite sides of the cell. If the cells have a sufficiently high threshold voltage, the cross-talk effect is reduced without sacrificing the resolution of the display.
Unfortunately, the cells in EPDs made using the typical electrophoretic materials and techniques currently available typically do not have a sufficiently high driving threshold voltage to prevent the undesired movement of particles. As a result, the EPDs constructed from these materials usually cannot achieve high resolution.
Cross bias is also a well-known problem for a passive matrix display. The voltage applied to a column electrode not only provides the driving bias for the cell on the scanning row, but it also affects the bias across the non-scanning cells on the same column. This undesired bias may force the particles of a non-scanning cell to migrate to the opposite electrode. This undesired particle migration causes visible optical density change and reduces the contrast ratio of the display.
A system having gating electrodes was disclosed in U.S. Pat. Nos. 4,655,897 and 5,177,476 (assigned to Copytele, Inc.) to provide EPDs capable of high resolution at relative high driving voltage using a two layer electrode structure, one of which layers serves as a gating electrode. Although these references teach how the threshold voltage may be raised by the use of gating electrodes, the cost for fabricating the two electrode layers is extremely high due to the complexity of the structure and the low yield rate. In addition, in this type of EPD, the electrodes are exposed to the solvent, which could result in an undesired electroplating effect.
Therefore, there is a need for a way to effectively raise the cell threshold voltage to avoid display performance degradation when a cross bias and/or cross talk condition may be present.
An electrophoretic cell generally has a top electrode layer which may have at least one row electrode and a bottom electrode layer which may have at least one column electrode. If there are no gating electrodes present, the electric field generated by the row and column electrodes would control the up/down movement of the charged particles. The present invention is directed to an improved design, which has at least one in-plane gating electrode. The gating electrodes may be on the top electrode layer, on the bottom electrode layer or on both layers.
It should be appreciated that the present invention can be implemented in numerous ways. Several inventive embodiments of the present invention are described below.
In one embodiment, the electrophoretic display comprises electrophoretic cells filled with charged particles dispersed in a dielectric solvent. Each cell is positioned between a top electrode layer and a bottom electrode layer. The top electrode layer comprises at least one driving electrode positioned over more than one cell. The bottom electrode layer comprises at least one driving electrode positioned under more than one cell. The display further comprises at least one in-plane gating electrode, located in either the top layer or the bottom layer.
The gating electrode(s) provide a gating effect, which raises the effective threshold voltage to prevent the undesired movement of the charged particles in the cells. In addition, the design of the present invention can be manufactured using low cost materials by efficient processes.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures, which illustrate by way of example the principles of the invention.